1. Field of the Invention
The present invention refers to an optical fiber for transmitting high optical power and of the kind comprising a central portion, the core, adapted to conduct radiation, and a layer, the cladding, surrounding the core and at least one covering surrounding the cladding and adapted to give an improved mechanical stability to the fiber and to damp scattered radiation.
2. Description of the Prior Art
The spreading of optical radiation in a material or its reflection in interfacial surfaces between materials are described by the refractive index of the materials. The refractive index consists of a real portion n and an imaginary portion k. If the difference in refractive indexes is very high between two materials there will be a considerable reflection in the interfacial surface between the materials.
The imaginary portion k describes the absorption in the material, so that a great value of k gives a strong absorption.
For materials having a low damping (a negligible k) a total reflection can be obtained in an interfacial surface between two materials. If the material in which the radiation spreads has a higher refractive index n than the material against which the radiation is reflected. A condition for this is that the angle of incidence towards the surface is sufficiently small. In interfacial layers where the refractive index is not suddenly changed but is changed from a high to a low level over a certain distance, the radiation will turn in this interfacial layer. Also in this case a total reflection can be obtained towards the interfacial layer.
In an optical fiber this is used for keeping the radiation limited to the core of the fiber. A condition for this is that one choses the refrective index of the core n.sub.1 somewhat higher than for the cladding n.sub.2, n.sub.1 &gt;n.sub.2, or that the core has a varying refrective index with a maximum in the central part of the core.
When the radiation enters the fiber it is focused towards the core. A part of the radiation will, however, always leak out into the cladding. Further, there will always be a leakage of radiation along the entire length of the fiber, caused by different types of inhomogeneities in the core material. This leak radiation is always a disadvantage in the transmission of information, and therefore this leak radiation conducted further along the fiber should be avoided. This can be achieved by applying a radiation absorbing covering, a cladding covering of e.g. silicon rubber, outside the cladding. The cladding covering will absorb all leak radiation which for any reason will reach the cladding. If one tries to use such a fiber for transmission of high optical power, e.g. laser radiation, for working of material, the radiation which has leaked out into the cladding will be absorbed in the cladding covering and melt and/or ignite the covering.
Optical fibers for transmitting high power have up to now been used commercially only in the medical field. These have mostly been argon ion lasers with a power of some W and continuosly radiating Nd-YAG-lasers of up to 100 W, where YAG denotes a crystal with a chemical composition Y.sub.3 Al.sub.5 O.sub.12.
In certain cases the fibers have been designed in a special way in order to avoid damage at the end surfaces. It is for example known to design the core with an increasing diameter at the ends, see e.g. U.S. Pat. No. 3,843,865. The quality of the fibers today is however so high, that damage at the end surfaces is not a great problem. In cases where the fibers have not been designed in any special way relatively thick fibers, e.g., 0.6 mm in diameter, have been used in order that all radiation enters the cores of the fibers and minimize the leak radiation. This involves the disadvantages that the radiation cannot be focused as well as when a thinner fiber is used. The fiber will also be difficult to handle. For medical purposes this is often no disadvantage since there are mostly "large" surfaces (of the size 1 mm) which are radiated.
In the Swedish Patent Specification No. 406 816 there is described a light conductor with improved mechanical strength. According to one embodiment the light conductor comprises a fluid core with three coverings, wherein the refractive index of the second covering is greater than that of the first (inner) covering. The second covering comprises a strongly light absorbing material. This light conductor cannot be used for transmitting high optical power, as the leak radiation absorbed by the covering quickly would melt and/or ignite the covering. The publication does not describe the problems associated with the transmission of high optical power, but the object of this known light conductor is only to provide an increase of the mechanical strength.
The German Offenlegungsschrift No. 2 442 859 describes a light conducting fiber provided with a filter in the form of several dielectric layers in order to selectively remove certain radiation. These layers have a thickness of the size wave-length and are attached directly against the core of the fiber. It can thus be regarded that they replace the cladding in a convention optical fiber. The layers are very sensitive to high power and this light conducting fiber is not intended to transmit high optical power.